1. Field of the Invention
Marek's disease (MD) is a herpesvirus-induced lymphoproliferative disease that occurs naturally in chickens, and has been one of the leading causes of economic losses in the poultry industry. Since the advent of the turkey herpesvirus vaccine (HVT), newly hatched chicks have been routinely inoculated against the disease prior to being placed in the brooder houses. Although HVT vaccine is generally quite effective, occasionally inoculated flocks experience heavy MD losses. Several factors may be responsible for vaccine failure, including the possibility that chickens become exposed to virulent MD virus too soon after vaccination before they have developed adequate immunity. Similarly, neonate poultry chicks are susceptible to other common poultry pathogens despite first-day inoculation with vaccines. This invention relates to the control of disease in avian flocks by means of embryonic vaccination.
2. Description of the Prior Art
Taylor et al. (U.S. Pat. No. 2,851,006) teaches a method for increasing the hatch rate of bacterially infected eggs by means of in ovo treatment with a suitable bacteriophage. The phage is introduced to the interior of the egg prior to incubation by any of a variety of techniques including hypodermic syringe, pressure differential in a dipping fluid, and jet spray. By virtue of this technique, disease agents present in the extraembryonic membranes and fluids can be controlled by direct action of the inoculant. In U.S. Pat. No. 3,120,834, Goldhaft et al. expands the application taught in Taylor to a variety of substances including antibiotics, sulfonamides, vitamins, enzymes, nutrients, and inorganic salts. These agents in a liquid carrier are introduced through the shell prior to incubation by means of vacuum impregnation. Nicely et al. (U.S. Pat. No. 3,256,856) offers an improvement to the method of Goldhaft et al. in providing one or more holes in the egg shell for facilitating penetration. The hole is made in the air cell end of the egg, not extending beyond the inner shell membrane. The commercial practicality of the vacuum impregnation technique is limited by the unreliability of obtaining a uniform treatment and the economic unfeasibility of charging the dipping vats with expensive vaccines.
Miller teaches in U.S. Pat. No. 4,040,388 an automated method and apparatus for injecting embryonated eggs prior to incubation with a variety of substances. Injection is made into the albumen at the small end of the egg adjacent to the inner surface of the shell. The hole is thereafter sealed by coagulative cooking of the surrounding albumen. While the mechanics of the system are apparently functional, Miller fails to appreciate that the avian embryos prior to incubation have insufficient immunocompetence to benefit from the several vaccines contemplated for injection. Moreover, the vaccines are susceptible to inactivation during the heat coagulation step, and the albumen has an inhibitory effect on the transport of the inoculant to the embryo at the egg's opposite end.